1. Field of the Invention
The present invention relates generally to aircraft and projectile flight controls, and more specifically, to rotatable, non-circular forebody shapes for yaw control at high angles of attack.
2. Description of the Related Art
Interest continues in improving the performance of fighter aircraft and missiles at high angles of attack. Fighter aircraft engaged in aerial combat must make aggressive maneuvers, particularly during one-on-one combat with other fighter aircraft. These aggressive maneuvers frequently require deceleration to low speeds and high angles of attack.
A major factor which limits the effectiveness of current fighter aircraft at high angles of attack is the degradation of rudder yaw control as the angle of attack increases. The level of yaw control provided by a conventional rudder decreases as the angle of attack increases because the vertical tail becomes immersed in a low-energy stalled wake generated by the wing, but the magnitude of yaw control required increases due to the higher yaw rates needed to coordinate rolling maneuvers at higher angles of attack. Reduced maneuverability of the aircraft results because the amount of yaw control required exceeds the amount available, thereby restricting the aircraft to an angle of attack substantially less than the angle for maximum lift.
On the other hand, the forebody of an aircraft at high angles of attack remains in flow undisturbed by the wings and creates a pair of powerful vortices. For most modern fighter aircraft, the moment arm from the forwardmost point of the forebody to the center of gravity is equal to or greater than the movement arm of the vertical tail or tails. The forebody is thus well-suited for placement of control surfaces which take advantage of the powerful vortex flow field and the long moment arm, and the forebody has become an increasingly important factor in aircraft design.
One attempt to control the vortex generated by the forebody at high angles of attack uses jet blowing to influence the direction of the naturally occuring vortex asymmetry in order to effect yaw control. While research has shown promise for jet blowing as a means of forebody control, jet blowing requires a source of high pressure air. This source normally comes from engine bleed air. Use of engine bleed air, however, can significantly reduce available engine thrust at a time in the maneuver when the aircraft needs maximum thrust.
Another method of generating moments at high angles of attack involves attaching nose strakes to the slender forebodies of missiles or aircraft. For example, longitudinally hinged forebody strakes provide large yawing moments at high angles of attack. A modern fighter aircraft, however, normally has a forebody with complex curvatures and cross-sections that vary along the length, making design and fabrication of longitudinally hinged forebody strakes technically difficult and expensive. Further, the radial location of hinged strakes determines the ranges of effectiveness with the angle of attack and the sideslip angle. Hinged strakes fixed at a predetermined radial location would thus limit the ranges of effectiveness. Longitudinally hinged forebody strakes may also adversely affect the performance of a radar unit mounted in the forebody.
Actuated forebody strakes are an alternative to longitudinally hinged strakes. This art, disclosed in U.S. Pat. No. 4,917,333, comprises an active control device that uses strakes which can be actuated, deflected, or rotated to provide the necessary direction and magnitude of yawing moment. Such a device, however, does not manipulate the naturally occurring vortices at the forebody but rather creates entirely new vortices.